1. Field of the Invention
This invention relates generally to the field of optical devices used to form an image of an extended scene having selectable spectral frequency bands. More specifically, the present invention is directed to a tunable multispectral filter for forming an image having selectable spectral frequency bands for subsequent spectral image detection and/or recordation of high resolution imagery covering large angular extent.
2. Description of the Related Art
Spectral imaging devices are classified generally as either interferometric or dispersive, according to the basic physical principle used to resolve the image into its spectral components, and in turn be viewed by an eye, recorded, and/or processed as desired. The desired objective of such devices is to form imagery of a scene containing extended objects subtending large angular extent with high spatial resolution in selectable spectral bands by selecting light or radiation in one narrow wavelength band. Depending on the specific application, a narrow wavelength band of the light radiation from the scene may be either passed or rejected from the total spectral band of the scene of interest. Spectral imaging devices of the type described find wide application in monitoring of earth resources, the atmosphere, agricultural and forestry resources, monitoring weather conditions from both aircraft and spacecraft, astronomy, target detection and recognition to name just a few.
Spectral imaging devices desirably have the capability of tuning to the center wavelength of a selected waveband, and be tunable to any wavelength in the broad spectral band of the sensitivity range of the instrument. It is desirable to have such devices capable of selecting or varying the bandwidth of the selected waveband. Accordingly, devices employing a set of filters with fixed spectral transmissive or reflective properties, implemented for example as a stepped mechanical filter wheel, does not provide such spectral agility. Thus sensing instruments having fixed filters have limited utility for many of applications.
Spectral imaging devices employing wavelength dispersive techniques have been taught to provide both imaging and spectral tunability. These devices generally resemble the classical form of imaging monochromators and spectrometers which makes use of a lens or mirror system to pass an image through a receiving input slit onto an output slit via a dispersive element, such as a diffraction grating or prism. With this arrangement the image of at the input slit is projected as a spectrum on the output slit plane, and, by selection of the position of the output slit it is possible to select a small specified wavelength band, the center wavelength of which can be varied by shifting the slit or dispersive element.
One example of the classical type employing an angular dispersion technique as just described is illustrated in U.S. Pat. No. 4,705,396, issued to Bergstrom, and particularly illustrated in FIG. 1A, herein. Bersgstrom includes an inner optical system, consisting of the classical two-slit monochromator described above with a diffraction grating to disperse the various wavelengths to be sampled, and an outer optical system, comprising an objective optics which forms an image of a scene on a recording means. The essential feature of Bergstorm's invention is to place the slits of the inner optical system at the aperture stop of the outer optical system. Bergstrom requires three optical stages and hence large volume. With the slits at the aperture stop of the outer optical system, spectral filtering is achieved by means of a spatial filter at the pupil rather than at an intermediate image plane. The use of a slit instead of the more common circular aperture at the stop of the objective optics greatly reduces the image quality of the system. This fact, together with the requirement that light incident on the diffraction grating be only slightly convergent or divergent, greatly limits the angular field-of-view over which Bergstrom's invention is capable of forming sharp imagery.
In contrast to Bergstrom, tunable spectral imaging devices employing longitudinally dispersive optical elements are described in U.S. Pat. No. 4,742,222, issued to Retfalvy, et al, and U.S. Pat. No. 5,479,258 issued to Hinnrichs, et al. FIG. 1B shows the imaging device of Retfalvy et al., and FIG. 1C shows the imaging device of Hinnrichs, which are fundamentally different from the classical monochromators and spectrometers which make use of angular dispersion. These latter mentioned imaging devices employ a simple single optical system having longitudinal chromatic dispersion, i.e., the various wavelengths incident on the optical system are dispersed along the optical axis. The method of Retfalvy et al. makes use of dispersive refracting materials to form the optical system, while the method of Hinnrichs et al. makes use of a single diffractive lens. The underlying principle of both methods is that the focal length of the optical system depends on observed image wavelength, and that at any set distance only a narrow range of wavelengths will be in sharp focus, as diagramatically illustrated in FIGS. 2B and 2C, corresponding to the imaging optics of FIGS. 1B and 1C, respectively. There are significant limitations with both methods, as described below.
Common to the imaging systems of both Retfalvy et al. and Hinnrichs is that they are intended to only detect and/or process point-like objects. That is, both imaging systems provide spectral content of objects which are not resolved in the image plane of the optics. Retfalvy et al. provide spectral tunability by translating a small aperture, such as an optical fiber, along the optical axis of the dispersive refracting lens, and at each focal distance the size of the aperture limits the range of wavelengths from the point-like object passed to an output image detector. In their method, the focal length varies in proportion with the wavelength; thus, shorter wavelengths are focused closest to the lens and longer wavelengths are focused farthest from the lens. However, since Retfalvy et al. rely on dispersive refracting materials, the size of the light distribution (blur spot) forming the image of the point object also varies with wavelength, with longer wavelengths having a larger distribution than shorter wavelengths as illustrated in FIG. 2B, requiring Retfalvy et al. to construct the size of the aperture in an adjustable fashion. Furthermore, the intended application of their device is wavelength demultiplexing in an optical communication system. Such a system presents only a single point source in a narrowly confined angular extent to the optical system. The method of Retfalvy et al. is not intended nor suited to cover a large angular extent containing a large number of point-like objects or a continuous scene. Retfalvy et al. is particularly limited by the need for an adjustable aperture and the restriction to a very small angular field-of-view. Hinnrichs et al. (FIG. 1C) relies on the dispersive properties of a diffractive lens in which the focal length varies inversely with wavelength, as illustrated in FIG. 2C. A two-dimensional electronic detector array is located in the focal region of the diffractive lens. Each detector (pixel) in the array in effect serves as the aperture as in the method of Retfalvy et al., but because a two-dimensional array is employed, the method of Hinnrichs et al can be applied to a larger angular field-of-view. Spectral tunability is achieved by translating the diffractive lens along the optical axis, bringing different wavelengths into sharp focus on the detector array. All wavelengths from the target and background are sensed simultaneously by the detector array, but the radiation in a narrow spectral band from a point target in sharp focus will project above the diffuse background signal, and the application of a spatial filtering algorithm in an electronic computer can be used to subtract the background signal, leaving only the signal from the point source in the sharply focused narrow spectral band.
The imaging device of Hinnrichs et al. has several disadvantages. Because all wavelengths are integrated simultaneously by the detector array and the desired spectral signal is extracted by frame subtraction, it works well only for a collection of well-separated point-like objects in an uncluttered background as one would find, for example, in aircraft or missiles at great distance against a sky background. Their method does not give clear spectral imagery of cluttered scenes or with extended objects having internal variations in brightness such as one finds in many applications of interest. Further, because the optical system consists of a single diffractive lens, image quality produced by the optics degrades with increasing field angle off the optical axis due to aberrations such as coma and astigmatism. Although this feature is not a severe limitation if one is interested only in viewing point targets against an uncluttered background, it is a serious limitation if imagery of cluttered scenes and extended objects is required. Because the focal length varies with wavelength, it is clear that the angular extent subtended by the image of an object will also vary with the wavelength to which the system is tuned; equivalently, the magnification of the system will vary with wavelength. For example, the apparent size of an object (it's image) will be larger when the sensor is tuned to red light than when tuned to blue light.
Accordingly, in view of the foregoing limitations of spectral imaging devices of the prior art, there is a need for imaging devices which provide a spectrophotometric imaging device capable of obtaining high quality imagery of scenes and extended objects which can subtend a wide angular field-of-view, and which can exhibit large variations in internal radiance or luminance, and to provide the imagery by tunable spectral composition in ultraviolet, visible, or infrared radiation.